A type of resistor in common use involves an insulating substrate core to which has been added a metal film. The core is usually composed of a ceramic or glass substance to which is added a nickel-chromium alloy (nichrome) or nickel-chromium alloyed with one or more other elements which is evaporated or sputtered onto the substrate. A nichrome film is used in resistors because of its stability and near-zero temperature co-efficient of resistance (TCR) in the resistors.
It is common to add aluminum to these nichrome films in order to achieve a better TCR. Work with superalloys has indicated that the addition of several percent of aluminum to nichrome superalloys allows a surface oxide to form which consists mainly of Al.sub.2 O.sub.3. This oxide scale provides greater protection against impurities and corrosion. However, the Al.sub.2 O.sub.3 oxide spalls at high temperatures. It has further been found that relatively minor additions of rare earth or transitional metals improve the oxidization resistance of the nichrome-aluminum superalloys. Other studies have suggested that common impurities which might invade the film pull electrons away from metal atoms and prevent these electrons from contributing to the stronger metallic-type bonds across grain boundaries.
These studies have not translated easily to work with nichrome films as opposed to superalloys. Research connected with superalloys has not involved electrical resistors. For instance, a patent was issued to NASA in July 1982, involving work to improve superalloys. In that U.S. Pat., No. 4,340,425, zirconium was added to improve the performance of the superalloys with about 0.13% weight optimal and a range of 0.06 to 0.20% weight effective. Work by this inventor discloses that nichrome films require much higher percentages in order to obtain the desired effects, with improvements noted for percentages from about 1.0% to 6.0%, with the optimum around 3.0%. Resistor films also differ considerably from superalloys in their basic makeup. For example, resistor films have a chromium content of 30% or higher, whereas the superalloys usually have a chromium content of 10 to 20%. It is necessary to add 1.0% or more of transitional metals or rare earth elements to obtain results with nichrome film, whereas additions of a fraction of a percent seem optimum for superalloys.
The theory has been developed in the discovery of this invention that addition of the elements listed in this patent improve the resistance of the oxide to corrosion and/or enhance Al.sub.2 O.sub.3 scale and nickel-chromium-aluminum adherence and stability of the scale. All of the elements are oxygen-active and are also sulfur-active. Elements with a large atomic radius as compared to nickel, which are nearly insoluble with nickel, and which are oxygen-active are also candidates for improving Al.sub.2 O.sub.3 adherence and stability and, thus, nichrome film stability.
It is the object of this invention to provide a nichrome film or metal film substitute with improved electrical stability on high temperature storage or high power operation or a combination of the two. It is the further objective of this invention to provide greater protection from impurities and inhibit oxide spalling.